High vacuum cold trap



Oct. 27, 1970 B|LL$ EI'AL Re. 26,973

HIGH VACUUM COLD TRAP Original Filed March 13, 1964 2% a a 13% MwM M 4fi w 6 a 5 7 a 7 5 W Z W 1 W a w 4 w w n n.0, J u 2, m 3 w 8 i, a, x 5 32% Z M Z 4, M g. L

INVENTORS Z fl/V/z 227/ A fiaez/v BY 2 i M ATTORNEYS United StatesPatent Ofifice Re. 26,973 Reissued Oct. 27, 1970 26,973 HIGH VACUUM COLDTRAP Daniel G. Bills and Keith A. Warren, Boulder, C010,, as-

signors to Granville-Phillips Company, Boulder, Colo., a corporation ofWashington Original No. 3,304,731, dated Feb. 21, 1967, Ser. No.351,689, Mar. 13, 1964. Application for reissue July 23, 1968, Ser. No.754,507

Int. Cl. B01d 5/00 U.S. Cl. 62-555 14 Claims Matter enclosed in heavybrackets appears in the original patent but forms no part of thisreissue specification; matter printed in italics indicates the additionsmade by reissue.

This invention relates to devices for preventing the return of diffusionpump fluids into a vacuum system and, more specifically, to high vacuumcold traps.

Diffusion pumps provide one of the fastest and most efficient ways ofevacuating systems designed to operate at extremely low pressures andthese pumps are, therefore, widely used for this purpose. One seriousproblem results from the use of this type of pump and that is thereturn-flow of the pumping fiuid to the system being evacuated. Toprevent return-flow and enable the diffusion pump to take the systemdown to its ultimate possible pressure, extremely efficient trappingmust be provided ahead of the pump. Obviously, the design andeffectiveness of the trap becomes more critical as the system pressuresmove into the ultra-high vacuum range and the systems themselves must bekept free of contaminants.

Trapping alone can be achieved rather simply provided one is satisfiedwith minimal conductance. The optimum trap, on the other hand, providesboth maximum trapping and maximum conductance at reasonable cost. Theprior art attempts to design such a trap have met with little success,however.

The available traps all have a common deficiency and that is theirfailure to prevent oil from passing from the pump back into the system.This can occur in any one of several ways. For example, if a roomtemperature path exists between the ends of a. trap, the oil will slowlycreep along this warm surface and eventually evaporate into the cleansystem. Morecules of oil can also bounce oft of warm surfaces inside thetrap and rebound into the clean system thus contaminating same. It iseven possible for one oil molecule to bounce off another and pass intothe clean system without ever contacting any surface of the trap unlesswell-designed battles are provided and the latter are likely to lowerconductance.

In general, surface migration and contamination resulting from moleculesbouncing off the trap walls can be effectively prevented by maintainingall interior surfaces at the temperature of liquid nitrogen orthereabouts. Intermolecular collisions are also no problem providedthese colliding molecules must rebound against one or more cold wallsurfaces before they can emerge from the trap. Actually, at very lowpressures, molecule-molecule collisions occur infrequently andmolecule-wall collisions predominate.

One of the major sources of system contamination is the release ofpreviously trapped molecules rather than the direct back-fiow thereof inone of the ways just described. Nitrogen and oxygen molecules, forinstance, are relatively loosely held on a liquid nitrogen cooledsurface and a change in wall temperature of only a degree will releasesome of these trapped gas molecules back into the system. Most of theprior art traps are designed with the idea in mind of keeping allinterior wall surfaces at the same temperature when, in accordance withthe teaching of the present invention, this is not nearly as importantas preventing the various cooled surfaces from changing temperature eventhough they are not all at the same temperature. It is also important toprevent warm gases from coming into contact with the refrigeratedsurfaces of the trap because this causes localized heating, yet, mostcommercially available traps are so designed that this takes place whilethe trap is being filled.

From the above it will be apparent that a well-designed trap shouldprovide a structure whereby the temperature of all trapping surfacesremains constant or nearly so over extended periods of use and theconductance path by which heat is transferred to the coolant arearranged to conserve the latter as long as possible. These and otherdesirable ends are accomplished in accordance with the teaching of theinstant invention by providing a coolant reservoir with a doubleinterior Wall that emerges as an exterior wall along a junction quiteremote from any surface in direct contact with the coolant thusproducing a long heat leak path that prevents rapid evaporation. Moresignificant, however, is the feature of the doublewalled reservoir thatkeeps the various areas of the trapping surface at a substantiallyconstant, but not necessarily the same, temperature until the coolantlevel in the reservoir falls below the juncture between this exposedinterior wall and the covered interior wall in direct contact with thecoolant. Also, the location of the fill opening in combination with thisdouble-Walled coolant reservoir is such as to prevent warm room air fromcoming into direct contact with the trapping surfaces.

It is, therefore, the principal object of the present invention toprovide a novel and improved high vacuum cold trap.

A second object is the provision of a device of the type aforementionedthat combines the desirable characteristics of essentially completetrapping with near maximum conductance.

Another objective is to provide a cold trap designed to maintain alltrapping surfaces at essentially constant temperatures during the entireperiod the system is being evacuated.

Still another objective of the invention herein claimed is the provisionof a liquid nitrogen cooled trap that conserves the coolant severaltimes longer than prior art traps.

An additional object is to provide a cryogenic vessel in which the lossof coolant and accompanying decrease in fluid coolant level has littleor no effect on trapping etiiciency.

Further objects of the invention are the provision of a cold trap of thetype used with diffusion pumps that is simple to construct. inexpensive,compact, versatile, easy to operate, relatively rugged and one capableof preventing back-flow of contaminants into the clean system to thedegree required for ultra-high vacuum operation.

Other objects will be in part apparent and in part pointed outspecifically hereinafter in connection with the description of thedrawings that follow. and in which:

FIGURE 1 is a diametrical section showing the details of construction ofthe high vacuum cold trap of the present invention; and

FIGURE 2 is a transverse section taken along line 22 of FIGURE 1.

FIGURE 3 is a block diagram of an illustrative embodiment of a system inaccordance with the invention.

Referring now to the drawings for a detailed description of the coldtrap that forms the subject matter of the present invention and whichhas been broadly designated by reference numeral 10, it will be seen toinclude an outer shell 12 formed from upper and lower substantiallyidentical open-bottomed cup-shaped sections 14 and 16 that are fastenedtogether along seam 18 formed by adjoining out-turned flanged edges 20.The upper edge 22 of upper section 14 that borders the central opening24 therein is secured to the external annular shoulder 26 of tubularelement 28, said shoulder defining the junction between the thick wallsection and the thin wall section 32. Tubular element 28 forms theintake into the trap and is connected into the system to be evacuated byconventional means not shown.

The edge 34 of the lower section 16 which corresponds to edge 22 of theupper section 14 is similarly secured to a flange 36 that borderscentral opening 38. This flange is fastened in a manner well known inthe art to a diffusion pump which has also been omitted from thedrawings because it forms no part of the present invention.

At the lower extremity of tubular element 28 is positioned an innershell 40 having a shape similar to that of outer shell 12 except that itis about half as high and is substantially smaller in diameter. Shell 40does, however, include two open-bottomed identical cup-shaped sections42 and 44 fastened together along their flanged adjoining edges 46 toform joint 48. Both upper section 42 and lower section 44 includecentral openings 50, the one in the upper section receiving the lowerextremity of tubular element 28 as shown.

This inner shell 40 constitutes the primary trapping surface and isfitted with a circular refrigerated baffle assembly 52 supported in thecenter thereof between openings by inlet and outlet conduits 54 and 55open into the interior of the coolant reservoir. The diameter of thebafiie assembly 52 is approximately the same as openings 50 in the innershell and it is readily apparent that no contaminating oil moleculescould pass directly from the diffusion pump side of the trap whereflange 36 is located upwardly into tubular element 28 without strikingeither the battle assembly or one of the walls of inner shell 40 thatconstitute the primary trapping surfaces.

Now, the most important feature of the present invention lies in thestructure and location of the coolant reservoir in relation to the partspreviously described. This coolant reservoir has been broadly designatedby reference numeral 56 and will be seen to include as a part thereofinner shell 40 together with a tubular wall portion 58 and anintermediate shell 60 that is, once again, formed by substantiallyidentical upper and lower openbottomed cup-shaped sections 62 and 64that are con nected together at their adjacent flanged edges 66 to formjoint 68. This intermediate shell 60 is somewhat smaller than the outershell leaving an open space or cavity 70 therebetween which iscustomarily evacuated to better insulate the coolant reservoir 56 beforethe coolant is placed therein. This intermediate shell also lies inspaced relation on the outside of the inner shell 40 to define thecavity 72 into which the coolant is placed.

Tubular wall portion 58 of the coolant reservoir encircles the thin wallsection 32 of tubular element 28 and lies in spaced relation on theoutside thereof except where these two concentric tubes are connectedtogether along their lower edges to form joint 74. The edge 76 of theupper section 42 of inner shell 40 that borders the central opening 50therein is also connected to the lower edge of tubular wall 58 at joint75.

The upper edge of wall 58 is joined to the upper section 62 of theintermediate shell 60 along seam 77. The lower edge of the lowersections of both the inner and intermediate shells ars similarly joinedtogether at 78 to produce a mass spectrometer leak-tight seam as are allof the seams and joints previously described. Thus, an examination ofFIGURE 1 will reveal that the coolant reservoir 56 comprises theinterior surfaces of the intermediate shell and the exterior surfaces ofthe inner shell and outer tubular wall member 58.

Adjacent surfaces of the intermediate and outer shells at the top of thecoolant reservoir are provided with fill openings 80 that areinterconnected across cavity 70 by a bellows 82. This bellows is, ofcourse, provided with a suitable gas-flow restriction device that hasnot been illustrated.

With specific reference to FIGURE 1, the baffie assembly 52 will be seento include a second coolant reservoir 86 formed by a pair of slightlyconvex circular plates 88 and 90, joined together along their peripheraledges in a manner to define a coolant cavity 92 therebetween. As themain reservoir 56 is being filled with coolant, the coolant also flowsinto the coolant cavity 92 of the bafiie assembly through conduit 55 andis vented through conduit 54. This baffie assembly also includes a pairof plane circular disks 94 and 96 of substantially the same diameter asthe second reservoir that are spot-welded to the top and bottom of thelatter at the center where they make essentially point contact with oneanother. These disks 94 and 96 are quite thin and cooperate to definesecondary trapping surfaces that are refrigerated and maintained at asubstantially constant temperature.

In use, the upper end of tubular element 28 represented by the thickenedwall section 30 is connected into the high vacuum system being evacuatedwhile flange 36 is similarly attached to a diffusion pump resulting in aclosed leak-tight system capable of sustaining low pressures in the highand ultra-high vacuum range as these terms are commonly used. For bestresults, a substantial vacuum is drawn in the system before thecryogenic liquid is introduced as this evacuates cavity 70 between theouter and intermediate shells providing better insulation for thecoolant reservoir by preventing heat transfer by air conduction betweenthese shells. Once this has been accomplished, the cryogenic liquid isintroduced into the coolant reservoir through the fill openings 80 untilthe reservoir is full, whereupon, the reservoir is loosely capped. Thecoolant lies in direct contact with the inner shell and outer tubularwall 58 but does not contact the thin section 32 of inner tubularelement 28, the latter surface being cooled by conduction across joint74 which lies about halfway down in the reservoir and is also underneathby far the greater volume of coolant when the reservoir is full. Now,thin wall section 32 of the inner tubular element 28 constitutes thesecondary trapping surface of the unit and, while all areas of thisthin-walled section will not be at exactly the same tem pcrature due tothe closer proximity of the lower part to the cooled joint 74, thesetemperatures will not change significantly until the fluid level of thecoolant is reservoir 56 falls below this scam or joint where the innerand outer tubular walls and inner shell all meet. Conversely, a longheat conductivity path is provided by thin-walled section 32 thatrequires heat reaching the coolant by direct conduction to enter atshoulder 26 and travel all the way down this inner tubular wall beforebeing transferred to the reservoir. Of course, wall sections 58 and 32have an evacuated space 84 therebetween that effectively prevents thetransfer of heat therebetween that does not migrate across joint 74, theheat transfer by radiation being minimal.

The above-described simple, but novel, features contribute to theproduction of a cold trap which provides maximum trapping withoutmaterially reducing conductance. The cooled primary and secondarytrapping surfaces greatly decrease the chances of an oil moleculerebounding and it can be shown that an average molecule must make aboutseven such collosions with a cooled surface to pass clear through theinstant trap thus decreasing the probability that one will do so to thepoint where it becomes negligible.

Referring to FIGURE 3, the cold trap 10 is shdwn in operative relationto a typical vacuum system 100 and a diffusion pump 102.

Having thus described the several useful and novel features of the highvacuum cold trap of the present invention, it will be apparent that themany worthwhile objectives for which it was designed have been achieved.Although but a single specific embodiment of the invention has beenillustrated and described, we realize that certain changes andmodifications therein may well occur to those skilled in the art withinthe broad teaching hereof; hence,

it is our intention that the scope of protection alforded hereby shallbe limited only insofar as said limitations are expressly set forth inthe appended claims.

What is claimed is:

1. A trap for use between a difiusion pump and a closed system beingevacuated thereby for preventing the return of pumping fluids into thesystem which comprises: an outer gas-impervious shell having an inletopening in the top and an outlet opening in the bottom, a tubular neckattached to the outer shell in leak-tight relation and in position toprovide portions projecting both externally and internally from saidinlet opening, the externally projecting neck portion being adapted forconnection into the system to be evacuated and the internally projectingportion terminating in a free edge located inside the outer shell remotefrom the leak-tight joint therebetween, connecting means carried by theouter shell bordering the outlet opening therein adapted for leak-tightattachment to a diffusion pump, an inner gas-impervious shell suspendedwholly within the outer shell and in spaced relation thereto by means ofthe internally-located free edge of the tubular neck to which said innershell is fastened with a leak-tight joint, said inner shell having anopening in the top thereof communicating the interior of the neck and anopening in the bottom aligned with the outlet opening of the outershell, a battle assembly supported within the inner shell in spacedrelation to the walls thereof and between the top and bottom openingsadapted to block straight-line flow through said inner shell, a tubularwall having a bottom edge fastened to the free edge of theinternally-projecting tubular neck portion and projecting upwardlytherefrom in encircling spaced relation, said tubular wall having anupper edge terminating inside the outer shell in spaced relation to thelatter, an intermediate gas-impervious shell located between the innerand outer shells, said intermediate shell being fastened to the upperedge of the tubular wall and the lower end of the inner shell adjacentthe bottom opening therein so as to define with said inner shell andwall a leak-tight coolant reservoir adapted to receive a cyrogeniefluid, and said intermediate shell lying in spaced relation to the outershell so as to provide a chamber surrounding the coolant reservoir thatcan be evacuated through the outlet opening, and conduit meansconnecting the top of the coolant reservoir with the exterior of theouter shell for filling purposes, and wherein the joint between theinternally-projecting portion of the tubular neck and the bottom edge ofthe tubular wall is located below the major volume of the coolantreservoir and provides a conductance path extending from said jointlocated a substantial distance beneath the coolant level in thereservoir to the externally-projecting neck portion whereby theinternally-projecting neck portion is cooled.

2. The trap as set forth in claim 1 in which the annular space betweenthe inwardly projecting portion of the tubular neck and the encirclingtubular wall communicates the chamber surrounding the coolant reservoirbetween the intermediate and outer shells so that said annular space canbe evacuated along with said chamber to insulate the reservoir.

3. A gas and liquid impervious leak-tight trap for use between adiffusion pump and a closed system being evacuated thereby whichcomprises: first and second continuous annular wall elements joinedtogether along their upper and lower margins to define a hollow sealedcoolant reservoir having a central fiow passage therethrough, the firstwall element providing the inside reservoir surface and having anannular section of increased diameter adjacent the lower end thereofthat produces an enlargement in the bottom of the central flow passage,a third continuous annular wall element joined to the first annular wallelement at the top of the enlargement in the flow passage and extendingupwardly beyond said first wall element in spaced relation insidethereof, the top of the third annular wall element being adapted forconnection into a sealed system to be evacuated, a bafile assemblymounted in the enlargement of the central fiow passage in position toblock straightline flow through the latter while permitting unrestrictedfiow along the first wall element, a fourth continuous annular wallelement having its upper edge connected to the third wall element toform a sealed joint located in spaced relation above the juncture between the upper margins of the first and second wall elements, thefourth wall element having its lower edge adapted for connection to adiffusion pump, and said fourth wall element lying in encircling spacedrelation to the second wall element while coperating therewith to definean annular cavity capable of insulating the coolant reservoir whenevacuated, and means comprising a plug able conduit interconnectingopposed portions of the second and fourth wall elements at the top ofthe reservoir for introducing a cyrogenic coolant therein, wherein thebaflie assembly comprises, a second coolant reservoir formed by a pairof convex circular plates joined together along their peripheral edgesto define a coolant cavity therebetween, first and second substantiallyplanar disks attached to the top and bottom of the second reservoir atthe center thereof in a manner to make essentially point contacttherewith, an intake conduit connecting the first coolant reservoir withthe second adapted to provide gravity-flow into the latter, and anoutlet conduit connecting said second coolant reservoir with the firstadapted to provide gravity discharge into the latter.

4. A cold trap for use between a diffusion pump and a closed system tobe evacuated thereby which comprises: a gas-impervious shell havingsidewalls forming a gas flow passage therethrough with an inlet openingin the top for connection to a system and an outlet opening in thebottom for connection to a pump; a coolant reservoir having inner andouter walls surrounding said shell with that portion of the inner wallof the coolant reservoir surrounding the lower portion of said flowpassage being contiguous with the lower portion of said shell sidewallswhich define a lower contiguous wall portion while that portion of theinner wall of the coolant reservoir surrounding the upper portion of theflow passage is spaced from the upper portion of said shell sidewallswhich define an upper spaced wall portion serially connected to saidcontiguous wall portion, said coolant reservoir being of a configurationand having a normal coolant level such that the major portion of thecoolant volume is located above the junction between the lowercontiguous wall portion and the upper spaced wall portion of said flowpassage, said upper wall portion of said shell, spaced from said innerreservoir wall, extending upwardly from said contiguous wall portion andterminating above the normal liquid level of coolant in the reservoir toprovide a heat conductance path from said contiguous wall portion alongthe entire length of the spaced wall portion to the upper extremity ofthe trap; conduit means for introducing a cyrogenic coolant into saidreservoir; and, baflle means positioned in said shell within the flowpassage wherein a substantial portion of the total surface area of saidflow passage forms a portion in common with the surface of said coolantreservoir and wherein the remaining portion of said total surface areaof said fiow passage is spaced from the surface of said reservoirdefining the major portion of the total volume of said coolantreservoir.

5. A cold trap for use between a pump and a system to be evacuated, saidtrap comprising:

an outer shell connected between said pump and said system; a coolantreservoir disposed within the outer shell; means for supporting thereservoir extending between said outer shell and said reservoir andconnected to said reservoir below a major volume of the reservoir;

and said coolant reservoir being disposed between the outer shell andthe extended supporting means.

6. A cold trap as in claim where the connection between said supportingmeans and said reservoir is disposed approximately at the bottom of saidsupporting means and a point substantially removed from the bottom ofsaid reservoir.

7. A cold trap as in claim 5 where said supporting means includes aninwardly projecting, tubular neck connected to said shell.

8. A cold trap as in claim 7 where said coolant reservoir is annular inshape and comprises inner and outer walls, said tubular neck beingconnected to said inner wall.

9. A cold trap as in claim including conduit means connecting the top ofthe coolant reservoir with the exterior of the outer shell for fillingpurposes.

10. A cold trap as in claim 9 including bafi'le means disposed withinand supported by the inner wall of said reservoir.

II. A cold trap for use between a pump and a system to be evacuated,

an outer gas-impervious shell having an inlet opening in the top forconnection to said closed system and an outlet opening in the bottom forconnection to said pump;

a tubular neck attached to the outer shell in leaktight relation andprojecting internally into said shell; said neck terminating in a freeedge located inside the outer shell remote from said leak-tight joint;

a coolant reservoir suspended wholly within the outer shell and inspaced relation thereto so as to provide a chamber that can be evacuatedthrough the outlet opening of said outer shell, said reservoir havinginner and outer walls, the inner wall of said reservoir being connectedto said free edge of the tubular neck;

wherein the connection between the tubular neck and the reservoir islocated below the major volume of the reservoir and wherein aconductance path is provideo from said outer shell to said connectionthrough said tubular neck whereby the temperature gradient along saidneck is maintained substantially constant as the coolant level in saidreservoir is decreased.

12. A cold trap as in claim 11 where the connection between said tubularneck and said reservoir is disposed approximately at the bottom of saidtubular neck and a point substantially removed from the bottom of saidreservoir.

13. A cold trap as in claim 5 where said supporting means issubstantially closed to prevent any trapped gases from directly escapingfrom the major volume portion of the reservoir past said support meansinto said sys tent to be evacuated as the coolant level in saidreservoir decreases.

14. A cold trap as in claim 11 where said tubular neck is substantiallyclosed to prevent any trapped gases from directly escaping from themajor volume portion of the reservoir past said support means into saidsystem to be evacuated as the coolant level in said reservoir decreases.

References Cited The following references, cited by the Examiner, are ofrecord in the patented file of this patent or the original patent.

UNITED STATES PATENTS 1,845,247 2/1932 Davidson 6 2-555 2,594,232 4/1952Stockstill 165-171 3,044,275 7/1962 Drewes -269 X 3,081,068 3/1963Mi1leron 55-269 X 3,137,551 6/1964 Mark 55-269 3,144,756 8/1964 Arnoldet a] 55-268 X 3,166,915 1/1965 Kiipping 62-169 3,188,785 6/1965 Butler55-269 3,232,031 2/1966 Simons 55-269 3,321,927 5/1967 Hood 62-5553,416,326 12/1968 Stuifer 62-555 FOREIGN PATENTS 936,689 9/1963 GreatBritain.

WILLIAM J. WYE, Primary Examiner US. Cl. X.R.

